Step 1: Log in your Facebook account and click on the symbol on the upper right corner.
Step 2: Choose New Groups.
Step 3: Click on Groups then choose group you want to leave.
Step 4: To leave the group, click on and then click Leave Group.
Step 5: There will be a notification and choose Prevent other members from adding you back to this group, then click Leave group for confirmation.
Read more helpful info at fb login